I don't quite follow. Do you mean I should use the SPI version of the screen?
Since I am using NRF24 as a radio I thought that might not be possible, since it already occupies the Arduino Nano's SPI interface?

As you can hook up multiple devices to the same i2c bus, simply attach the OLED and two of those ADS1115 boards (apparently you can assign up to 7 different I2C addresses to the ADC boards...) to the I2C bus.

If you insist on connecting the analog sensors to the arduino and switch to a software I2C implementation on non-hardware I2C pins, you might simply extend the SSD1306Ascii library. It currently has classes for the SSD1306 over the AVR hardware i2c, another class for generic Wire implementations (not tailored to atmel chips; e.g. I'm using it on nrf51822), and the SPI devices.

Looking at the SoftwareI2CLibrary docs, it appears that they even provide a Wire interface (SoftWire.h), so you might even get away with the SSD1306AsciiWire class for the OLED.

If that does not work, you'll have to write your own software I2C implementation for the SSD1306Ascii class (e.g. takt SSD1306AsciiWire, rename it to SSD1306AsciiSoftI2C and adjust it to use the SoftwareI2C library for all I2C communication).)